1. Field of the Invention
The present invention relates to systems for allowing gas to be controllably injected into SMIF pods, and more particularly to a passive gas flow valve that is actuated between its closed and open positions by the weight of the pod.
2. Description of Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers, and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafers and/or wafer cassettes; (2) an input/output (I/O) minienvironment located on a semiconductor processing tool to provide a miniature clean space (upon being filled with clean air) in which exposed wafers and/or wafer cassettes may be transferred to and from the interior of the processing tool; and (3) an interface for transferring the wafers and/or wafer cassettes between the SMIF pods and the SMIF minienvironrnent without exposure of the wafers or cassettes to particulates. Further details of one proposed SMIF system are described in the paper entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Ulkich Kaempf, Solid State Technology, Jul. 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (.mu.m) to above 200 .mu.m. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half .mu.m and under. Unwanted contamination particles which have geometries measuring greater than 0.1 .mu.m substantially interfere with 1 .mu.m geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 .mu.m and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles become of interest.
In practice, a SMIF pod is set down on various support surfaces within a wafer fab, such as for example at a load port to a minienvironment, whereupon interface mechanisms in the load port open the pod door to allow access to the wafers within the pod. Additionally, a pod may be supported at a storage location while awaiting processing at a particular tool. Such storage locations may comprise a local tool buffer in the case of metrology or high throughput tools, or may alternatively comprise a stocker for storing large numbers of pods within a tool bay. A pod may additionally be positioned at a stand-alone purge station.
Whether a tool load port, local tool buffer, stocker or purge station, the support surfaces typically include registration or kinematic pins protruding upward from the support surface. In 200 mm pods, the support surface includes registration pins and guide rails which guide the pod into the proper rotational and translational position with respect to the pins. In 300 mm pods, a bottom surface of the pods includes radially extending grooves for receiving kinematic pins. Once the pod is positioned so that the grooves engage their respective kinematic pins, the grooves settle over the pins to establish six points of contact between the pod and support platform (at the grooves and pins) to kinematically couple the pod to the support platform with fixed and repeatable accuracy. Such a kinematic coupling is for example disclosed in U.S. Pat. No. 5,683,118, entitled "Kinematic Coupling Fluid Couplings and Method", to Slocum, which patent is incorporated by reference herein in its entirety. The size and location of the kinematic pins are standardized so that the pods of various suppliers are compatible with each other. The industry standard for the location and dimensions of the kinematic coupling pins are set by Semiconductor Equipment and Materials International ("SEMI").
Occasionally, it is advantageous to purge a pod of contaminants and/or particulates by creating a current flow through a pod to carry away the contaminants and/or particulates. It may also be beneficial to fill a pod with a non-reactive gas for longer term storage and certain processes. Additionally, it may be advantageous on occasion to provide the pod with a pressure higher or lower than ambient. In order to accomplish such purging, it is known to provide one or more valves within a pod which allow fluid flow to and/or from the interior of the pod. Inlet valves to the pod may be connected to a pressurized gas source to fill the pod with a desired gas, and outlet valves may be connected to a vacuum source to remove gas from the pod. The inlet and outlet valves may be used to purge the pod, including filling the pod with a desired gas, and/or providing a pressure differential within the pod relative to ambient. Such a system is disclosed in U.S. Pat. No. 4,724,874, entitled "Sealable Transportable Container Having a Particle Filtering System", to Parikh et al., previously incorporated herein by reference. Relative to systems which require opening of the pod for purging, valve systems require less components and space, and in general operate more efficiently.
Regardless of the mechanisms by which purging occurs, it is desirable to supply the purging gas from the gas source only when a pod is seated on the support surface. One reason is that the gas injected into the pod, typically nitrogen, can be harmful to fab personnel if released into the fab environment in large quantities. Therefore, it is preferable to activate gas flow when a pod to be purged is located on the support surface, and to deactivate gas flow once the pod is removed.
It is known to provide electrically controlled actuators and/or controllers, such as for example a mass flow controller, for activating, deactivating and regulating the flow of gas to a pod. It is also known to provide sensors for indicating the presence or absence of a pod on the support surface. Each of these systems require control circuitry for transferring sensor and power signals between the control system and the gas flow system. This complicates the control system, and also increases the likelihood of a malfunction at one or more of the purging stations.